High temperature methanation with molten salt-based catalyst systems

ABSTRACT

The reaction of hydrogen and carbon oxides (carbon monoxide and carbon dioxide) to form methane at temperatures above 500°C is promoted by carrying out the reaction in the presence of a molten metal salt-based catalyst system comprising a molten metal salt carrier selected from the class consisting of the halides and carbonates of alkali metals and alkaline earth metals and the halides of zinc, copper, manganese, cadmium, tin and iron, and mixtures thereof, melting below 1000°C; said molten salt having dispersed therein one or more catalytically active metals selected from the class consisting of iron, molybdenum, manganese, nickel, cobalt, zinc, titanium silver, copper and thorium in the form of finely divided elemental metals, metal oxides and/or metal carbides.

BACKGROUND OF THE INVENTION

This invention relates to catalytic methanation or hydrogenation ofcarbon oxides to methane and to a catalyst system for effecting same.More particularly, this invention is directed to a methanation processcarried out at high temperatures and a molten metal salt-basedheterogeneous catalyst system which is uniquely suited for such hightemperature methanation.

Catalytic methanation is a well-known reaction which is widely employedin the chemical and energy providing industries. Probably its mostwidespread current and potential application is in the treatment of thegaseous effluent from the gasification or partial oxidation ofcarbonaceous fuels with oxygen and/or water, e.g., steam-hydrocarbonreforming and partial combustion of liquid and solid carbonaceous fuels,to produce a hydrogen-rich gas for chemical synthesis, e.g., ammoniamanufacture, or petroleum refining, e.g., catalytic hydrocracking andhydrogenation, or to form a methane-rich gas having high BTU value andlow CO content for use in residential and industrial heating or powergeneration. In the former case, the gasification or partial oxidationeffluent, which typically contains substantial quantities of H₂, CO, CO₂and H₂ O as well as N₂ in cases where air is used as the oxidant source,is generally subject to a process known as the carbon-monoxideshift-conversion reaction prior to catalytic methanation. In this casethe CO-shift reaction converts a substantial quantity of the CO presentto H₂ and CO₂ by reaction with H₂ O in the presence of a catalyst andthe primary purpose of catalytic methanation is to remove smallquantities of CO which remain in the hydrogen-rich product gas byconversion to methane in order to avoid poisoning of downstreamprocessing catalysts. In the latter case, i.e., conversion of partialoxidation effluent gas to methane-rich gas, the gasification or partialoxidation effluent gas is subject to CO-shift to obtain the appropriateratio of H₂ to CO (usually 3 to 1) and the CO shift product gas is thensubject to catalytic methanation for conversion of carbon oxides andhydrogen contained therein to methane. In either case, the CO-shifteffluent gas is subject to an intermediate processing step to removesulfurous materials in cases where a sulfur-containing carbonaceous fuelfeedstock is employed since all commercially used methanation catalystsare highly sensitive to poisoning by sulfur compounds.

Because of the increasing demand for a high BTU, clean gas as an energysource in the United States and the acknowledged decreasing and finitenature of natural gas reserves in the United States as well ashappenings on the world scene which make energy self-sufficiencydesirable or even essential, there has been a dramatic increase ininterest in the manufacture of a clean, high BTU gas energy source whichwill meet pipeline standards by synthetic means from alternativecarbonaceous resources such as coal or heavy hydrocarbons. Many of themore attractive synthetic approaches which have been proposed are basedon gasification or partial combustion of the carbonaceous material, and,as indicated above, include catalytic methanation as part of theintegrated process scheme to upgrade the BTU value of the product gas toa level acceptable for pipeline gas applications. CO and H₂ have heatingvalues of about 300 BTU/ft³ whereas pipeline natural gas has a valueabove 1000 BTU/ft³. While a number of metallic species are known to beactive and selective methanation catalysts including, inter alia,nickel, ruthenium, cobalt, iron and molybdenum, their application to themanufacture of high BTU or pipeline gas has been less than satisfactoryfor several reasons which relate to the physical form of the catalystemployed and/or the nature of the methanation reaction, itself.

In the first place, the primary thrust of previous efforts to effectcatalytic methanation has been to utilize the active catalyst in solidform as a finely divided particulate on a refractory support, i.e.,nickel on alumina or kieselguhr being pre-eminent, or as an alloy in afixed or fluidized bed. These catalyst types are highly susceptible toinactivation via carbon deposition which can only be partially remediedby operation at undesirably high H₂ /CO mole ratios in the feed gas.Furthermore, methanation reactions with these catalyst systems generallymust be limited to temperatures below 400°C to avoid sintering anddeactivation of the catalyst and the highly exothermic nature of themethanation reaction itself provides severe operational difficulties incontrolling catalyst temperature in a fixed or fluidized bed at theselevels when the CO concentration of the feed gas is in the rangerequired for methane-rich gas manufacture. As an aside the use of thefixed or fluidized bed catalyst processing techniques also make itextremely difficult to recover any substantial quantity of the heatgenerated in the methanation for use in other phases of the process,e.g., the endothermic gasification in steam gasification of coal.Finally, the methanation reaction itself, is considered to be acombination of several reactions including the primary reaction (1)

    3 H.sub.2 +CO → CH.sub.4 +H.sub.2 O                 (1)

and secondary reactions (2) and (3)

    2 H.sub.2 +2CO → CH.sub.4 +CO.sub.2                 ( 2)

    4 h.sub.2 +co.sub.2 → ch.sub.4 +2h.sub.2 o          (3)

whose thermodynamic equilibria are such that the equilibrium yield ofmethane is adversely effected at high temperatures, i.e., above 500°C;reaction (2being a combination of reaction (1) and the water gas shiftreaction (4).

    CO+H.sub.2 O → CO.sub.2 +H.sub.2                    ( 4)

thus with conventional catalyst systems, methanations have been limitedto the lowest temperatures consistent with acceptable catalyst activityin part, because of catalyst instability at high temperatures, thehighly exothermic nature of the methanation reaction and the inabilityto effect an equilibrium shift towards methane, e.g., by absorption ofone of the reaction products, at high temperatures under practicalcircumstances. A good review of previous efforts in catalyticmethanation and the problems associated therewith can be found in G. A.Mill et al., "Catalytic Methanation," Catalysis Reviews, 8 (2), 159-210(1973).

Accordingly, it would be desirable if an active catalyst system formethanation at temperatures above 500°C could be developed which wouldminimize operational problems associated with high temperature operationof the solid, particulate catalysts of the prior art, e.g., carbondeposition, instability and heat removal, while at the same time somehowshifting the methanation equilibrium towards methane formation, e.g., byH₂ O absorption from the reaction mass, at these high temperatures. Thiswould be especially advantageous when catalytic methanation is utilizedin conjunction with, for example, steam gasification of coal for theproduction of methane-rich gas. This is because the coal gasificationreaction is high temperature but endothermic, thus requiring substantialinput of high temperature heat such as that which could be recoveredfrom an exothermic methanation reaction carried out a high temperatures.Furthermore, the reaction effluent from such coal gasification is manytimes already at or close to the thermodynamic equilibrium concentrationof methanation reactants in a high temperature methanation reactionscheme, due to the high steam concentration of the gaseous effluent, andas such cannot be catalytically promoted towards methane formationunless one of the reaction products, particularly H₂ O, is absorbed outof the reaction mass during or prior to methanation.

SUMMARY OF THE INVENTION

It has now been found that certain molten salt-based heterogeneouscatalyst systems are active in promoting the methanation of carbonoxides at temperatures above 500°C. These catalyst systems, whichcomprise molten metal salts or salt mixtures that melt below 1000°C butare stable under methanation conditions at temperatures above 500°C andcontain certain active metallic species for catalytic methanation infinely divided form, are less susceptible to carbon deposition thanprevious catalyst systems, exhibit little, if any, catalyst sinteringand inactivation at high temperatures and provide a superior means forreducing the problems associated with heat transfer from the exothermicmethanation reaction since the molten salt carrier functions as both aheat sink and a heat exchange medium for the reaction. Furthermore, themolten salt carriers of the invention are effective in absorbing waterfrom the methanation reaction mass at high temperatures, and, as such,function to shift the reaction equilibrium towards methane which isespecially critical in cases such as steam gasification of coal wherethe reactant mass approaches equilibrium concentration of reactants andreaction products at the high temperature methanation conditions,without an intermediate H₂ O removal step.

Accordingly, in its broadest aspects the instant invention provides aprocess for the preparation of methane from a gaseous reactant mixturecontaining hydrogen, carbon monoxide and/or carbon dioxide whichcomprises contacting said gas mixture in a reaction zone maintained attemperatures above about 500°C with a molten metal salt-based catalystsystem comprising a molten metal salt carrier selected from the classconsisting of the halides and carbonates of alkali metals and alkalineearth metals and the halides of zinc, copper, manganese, cadmium, tinand iron, and mixtures thereof, melting below 1000°C; said molten salthaving dispersed therein one or more catalytically active metalsselected from the class consisting of iron, molybdenum, manganese,nickel, cobalt, zinc, titanium, silver, copper and thorium in the formof finely divided elemental metals, metal oxides and/or metal carbides.Preferably, the gaseous reactant mixture is derived from the partialoxidation or gasification of a carbonaceous fuel and as such alsocontains a significant amount of water in the form of steam. Also withinthe scope of the instant invention are the novel molten salt-basedcatalyst systems, as hereinbefore described, which are active in thehigh temperature methanation reaction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The Catalyst

The high temperature methanation catalysts according to the inventionare heterogeneous catalyst systems comprising a molten metal saltcarrier which melts below 1000°C, but is stable under methanationreaction conditions at temperatures above 500°C, preferably about 500°to about 800°C, and which contains a uniform dispersion of a finelydivided metal, metal oxide and/or metal carbide of certain metallicspecies which exhibit catalytic activity for hydrogenation of carbonoxides or methanation. To function as an effective carrier in theheterogeneous catalyst system it is essential that the molten metal saltmelt be thermally and chemically stable under methanation reaction zoneconditions. That is, the molten salt or salt mixture must not behydrolyzed significantly with steam at temperatures above 500°C nor bereduced by the methanation reactant feed mixture under the conditionsprevailing in the reaction zone. Preferably, the metal salt or saltmixture melts below about 700°C and most preferably between about 100°and 600°C. Suitable metal salts or salt mixtures include the halides andcarbonates of alkali metals and alkaline earth metals and the halides ofzinc, copper, manganese, cadmium, tin and iron or mixtures thereof.Preferred metal salts or salt mixtures are mixtures of alkali metalhalides, mixtures of alkali metal carbonates, for example the ternaryeutectic of lithium, sodium and potassium carbonate; and the halides ofzinc, tin and iron, the latter metal halide having a minor amount ofalkali metal halide added thereto as a melting point depressant. Mostpreferred because of their availability and/or favorable effect on theactivity of the active metallic species in the catalyst system are thezinc halides, e.g., zinc chloride, zinc bromide and zinc iodide;mixtures of an iron halide with a sodium and/or potassium halide, e.g.,iron chloride-sodium chloride mix; and the ternary eutectic of lithium,sodium and potassium carbonate. The zinc halides are emminently suitableas molten catalyst carriers in the methanation of gaseous reactantmixtures containing substantial quantities of water such as that derivedfrom the steam gasification of coal since they absorb up to 90% or moreof the water present in a typical gasification effluent at methanationtemperatures above 500°C and thereby shift the methanation equilibriumtowards methane formation.

The active catalytic species in the molten salt-based methanationcatalysts according to the invention are finely divided metals, metaloxides and/or metal carbides of certain transition and actinide elementsand zinc which are uniformly dispersed in the molten carrier. While thisgroup of catalytically active metals does include certain metallicspecies which have heretofore been described as having catalyticactivity in methanation reactions, e.g., nickel and iron, it is notcompletely apparent that the instant dispersed solid catalysts functionin a manner equivalent to the prior art supported catalysts. This isbecause only certain of the known methanation catalysts are active inthe molten salt-based catalysts of the invention and a metal, zinc, notpreviously considered to have catalytic methanation activity is, infact, quite active as a methanation catalyst in the molten-salt basedsystems of the instant invention. The metals which show catalyticmethanation activity when dispersed as finely divided elemental metals,metal oxides and/or metal carbides in the molten salt-based catalysts ofthe invention include iron, molybdenum, manganese, nickel, cobalt, zinc,titanium, silver, copper and thorium. Of these catalytically activemetals, iron, zinc, manganese and molybdenum seem to provide the highestactivity and are preferred for that reason. Most preferred for reasonsof availability and activity are zinc and iron. As indicated above, thesolid metallic catalyst is present in the molten salt carrier as theelemental metal, metal oxide and/or metal carbide. Usually it is presentas a mixture of all three chemical forms with the elemental metal andmetal oxide being predominant. While the exact particle size of thefinely divided metal catalyst in the molten salt carrier is notconsidered critical to the operability of the invention, it appears thatthis average particle size diameter should not exceed about 0.5 mm, ifoptimum results are to be obtained.

The concentration of active metal catalyst dispersed in the moltensalt-based catalyst systems of the invention is not critical and willdepend, in part, on the concentration of carbon oxides and hydrogen inthe reactant gas; the purpose and extent of methanation desired, i.e.,production of methane-rich, high BTU gas or conversion of CO to methanein the production of a hydrogen-rich synthesis gas; and processconditions such as reactant feed rates, temperatures, pressures, etc.For most applications, the concentration of active metal catalystdispersed in the molten salt carrier will range from about 0.1% to about20% by weight of the total (catalyst + carrier) catalyst composition.Preferably, in cases where the catalyst is employed to upgrade partialoxidation or gasification effluent gas to methane-rich, high BTU gas,the catalyst concentration will range from about 5% to about 15% byweight of the total catalyst composition. As indicated previously thiscatalyst concentration may be composed of a single catalytically activemetal or a mixture of one or more of such metals. Preferred metalmixtures include zinc - iron, iron - manganese and zinc - manganese.

The active catalysts according to the invention may be prepared bysimple physical comminution of the metal catalysts, in the form ofrelatively pure elemental metals or metal oxides, to the desiredparticle size, e.g., hammer mill grinding, and subsequent addition ofthe ground metal or metal oxide powder to the metal salt carrier beforeor after heating to, or above, its melting point. Alternatively, theactive metal catalysts may be prepared by adding the catalyticallyactive metal to the metal salt before or after heating in the form of acompound or complex which will thermally and/or chemically decompose atmethanation temperatures into the desired metal and/or metal oxide.Examples of suitable metal compounds or complexes which will decomposeto yield the desired metal or metal oxide at methanation temperaturese.g., 500°C or above, include inorganic metal hydroxides and salts suchas nitrates and carbonates; organic salts or carboxylic acids such asformate, trifluoroacetate, butrate, 2-ethylhexanoate, lactate andcitrate; organo metal compounds and complexes as metalocenes, e.g.,ferrocene, metal carbonyls, e.g., iron pentacarbonyl, molybdenumhexacarbonyl, etc., and metal complexes such as those derived frompyridine and the metal acetate or 1-5-cyclooctadiene and the metalnitrate. It is also possible to prepare the dispersed metal or metaloxide in situ in the molten salt medium by adding two chemical compoundswhich will react, e.g., zinc bromide and sodium carbonate, in the moltenmedium to yield the desired active catalyst, e.g., zinc oxide. In thisalternative method of preparing the active catalyst, the thermallyand/or chemically decomposable metal compound or complex is preferablyadded to the metal salt carrier after the carrier has been heated to orabove its melting point (usually about 400°C or above) and the moltensalt or salt mixture is agitated for a time period ranging from 10 to120 minutes to allow the metal compound or complex time to decompose anddisperse in the molten medium. Preferred metal compounds or complexesfor use in this procedure include the metal carbonyls and metalocenes.In this case the gaseous reactants can be contacted by the moltencatalyst system prior to decomposition of all of the metal catalystcompound or complex precursor, since the precursor will continuouslydecompose and release active catalyst during the course of themethanation reaction.

The Process

As indicated above, the molten salt-based catalyst systems of theinvention have been found to be active in promoting methanationreactions at high temperatures, e.g., 500°C and above, heretoforeconsidered to be prohibitive due to catalyst deactivation andinstability, physical limitations on heat removal and methanationreaction equilibrium. Thus, in one aspect the instant inventioncontemplates a process for the preparation of methane from a gaseousreactant mixture containing hydrogen, carbon monoxide and/or carbondioxide which comprises contacting said gas mixture in a reaction zonemaintained at temperatures above about 500°C with a molten metalsalt-based catalyst system comprising a molten metal salt carrierselected from the class consisting of the halides and carbonates ofalkali metals and alkaline earth metals and the halides of zinc, copper,manganese, cadmium, tin and iron, and mixtures thereof, melting below1000°C; said molten salt having dispersed therein one or morecatalytically active metals selected from the class consisting of iron,molybdenum, manganese, nickel, cobalt, zinc, titanium, silver, copperand thorium in the form of finely divided elemental metals, metal oxidesand/or metal carbides.

The gaseous reactant feed to the catalytic methanation process of theinvention must contain at least some measurable amount of hydrogen andcarbon oxides (carbon dioxide and/or carbon monoxide). Preferably, thereactant feed mixture to methanation contains both hydrogen and carbonmonoxide at an H₂ :CO mole ratio of 2:1 with H₂ :CO reactant mole ratiosof 3:1 or more being most preferred. Gaseous reactant feed mixtureswhich can be suitably methanated with catalyst compositions of theinstant invention typically contain 10 to 99.9% H₂, 0.1 to 50% CO, 0 to20% CO₂, 0 to 70% H₂ O, 0 to 25% CH₄ and 0 to 70% N₂. Such gaseousreactant feed mixtures are quite suitably obtained from conventionalpartial oxidation or gasification of carbonaceous fuels such as, interalia, natural gas or normally gaseous hydrocarbons, e.g., C₂₋₄ saturatedand olefinic hyrocarbons; heavier hydrocarbon fractions includinggasoline, kerosene, naphtha, distillates, gas oils and residual oils;solid or semi-solid fuels including coal, oil shale, partial combustionsoot and bituminous residues from petroleum refining. Typically, thepartial oxidation or gasification effluent gas will be subject to aconventional CO-shift reaction to adjust the hydrogen to carbon monoxidemole ratio and an optional particulate removal step, e.g., one or morecyclone separators, prior to methanation according to the invention.However, at least the intermediate CO-shift step is not essential to thepreparation of a suitable gaseous feedstock for use in the inventionsince conversion of reactants to methane will still be effected to theextent that the stoichiometry of the reaction can be satisfied. Sinceall of the catalyst compositions according to the invention exhibit atleast some sensitivity to sulfur poisoning, it is essential that eithera desulfurized carbonaceous fuel be employed in the reactant gasgenerating process or that the reactant gas be subject to a conventionaldesulfurization procedure i.e., scrubbing with liquid or solidabsorbents for sulfur compounds (mainly H₂ S) prior to contact with themolten methanation catalysts of the invention.

One of the preferred applications of the catalytic methanation processof the instant invention is in the upgrading of methane-rich gas derivedfrom the partial oxidation or gasification of coal. Several coalgasification processes employing non-catalytic gasifiers in which coalis converted to a crude product gas containing principally CH₄, H₂, CO,H₂ O and CO₂ by high temperature reaction with steam and oxygen arequite well known, e.g., the Lurgi process, the Koppers-Totzek process,etc., and need not be detailed herein. A catalytic steam gasificationprocess for conversion of coal to methane-rich gas by reaction withsteam in the presence of certain alkali carbonate catalysts at about600°-750°C is described in U.S. Pat. No. 3,686,240 to Aldridge et al. Ingeneral, all of these coal gasification processes are endothermic in thegasification stage and produce a suitable gas feed mixture formethanation according to the instant invention even though the H₂ Ocontent may range as high as 50% by weight of the feed mixture, a valueat or near methanation equilibrium for conventional supportedmethanation catalysts. This feed mixture is suitable for hightemperature methanation with the instant catalysts because, as indicatedpreviously, the molten salt carriers of the instant invention have theability to absorb up to over 90% of the water present in the methanationfeed gas, thereby shifting the reaction equilibrium towards methane. Itis especially preferred that the instant methanation process be utilizedto upgrade the methane content of the gaseous effluent derived from acoat-steam gasification process such as that described in theaforementioned U.S. Pat. No. 3,686,240 since the endothermicgasification takes place at a temperature approximating the exothermicmethanation temperature. Thus, it is possible to utilize the heatgenerated by methanation via for example, heat exchange between themolten salt carrier and the gasification reaction zone, to at leastpartially satisfy the heat requirements of the gasification reaction.

Procedurally, the methanation process of the instant invention can besuitably effected by any conventional technique for intimatelycontacting a gaseous rectant feed with a molten or fluid catalyst. Suchtechniques include batch or continuous procedures wherein the gas isintroduced into the vapor phase of a reaction chamber or autoclavecontaining the molten salt-based catalyst and the catalyst is agitatedinto contact with the gas mixture. In the case of a batch reactionaccording to this procedure the product gas is merely withdrawn at theend of the reaction (measured by time and/or pressure drop) whereas inthe case of a continuous reaction the size of the reaction chamber andcatalyst to gas mass ratios in the reaction chamber are selected toallow sufficient gas-molten medium contact prior to continuouswithdrawal of product gas at some point in the vapor phase remote fromthe reactant feed port. Alternatively, the gas phase can be bubbledthrough a mass of molten catalyst in a reaction chamber or autoclave orpassed into countercurrent contact with the catalyst phase in avertically-oriented contacting column. In any case, the methanationreaction zone is maintained at temperatures above about 500°C while thegaseous reactant feed mixture is in contact with the catalyst.Preferably, the methanation reaction according to this invention iseffected at temperatures between about 500° and 900°C and mostpreferably between about 600° and 800°C. The pressures employed in themethanation reaction according to the invention generally range between100 psig and 1500 psig and preferably between 400 and 1200 psig.

The reaction or gas-molten salt contact time is not considered criticalto the operation of the methanation process of the instant invention,provided sufficient time is alloted to facilitate adsorption of waterinto the molten salt carrier and mass transfer and adsorption and thegaseous reactants on to the active catalytic species dispersed in themolten carrier. Accordingly, the reaction time should be at least about10 seconds with reaction times of about 10 minutes being a reasonablemaximum for practical operation. Preferably, the reaction time rangesfrom about 0.5 to about 5 minutes. In this regard the ratio of volume ofreactant feed gas to molten salt-based catalyst in continuous processesemploying the methanation catalysts of the invention may suitably rangefrom about 100 to about 5000 per hour (gas measured at STP).

EXAMPLE I

A number of molten salt-based catalyst systems according to theinvention were evaluated for catalytic methanation activity in acontinuous flow reaction system. In these experiments, the salt catalystcarrier (ca. 100 ml) was added to a methanation reaction chambercomprising a 300 ml Magna-Drive autoclave made of 316 stainless steel,Inconel or Hastelloy B, the salt carrier was heated to its melting point(usually 400°C or above) and the active catalyst was added to the moltensalt carrier under agitation as a metal and/or metal oxide powder or inthe form of a decomposable, metal compound or complex catalystprecursor. After heating to methanation reaction conditions (500°C orabove), the methanation reactant gas containing hydrogen and carbonmonoxide in a 2:1 mole ratio of H₂ :CO and trace amounts of carbondioxide and water was continuously introduced into the reaction chamberat a constant elevated pressure (about 400 psig) with the reactant flowrates being controlled at about 30 l/hr of reactant gas by adjusting thepressure drop across long lengths of stainless steel capillary tubing.During each reaction period the molten salt based catalyst was agitatedat about 1800 rpm and the reaction temperature was controlled betweenabout 500° and about 650°C with a single-point Thermoelectriccontroller. The reaction chamber pressure was controlled at a constantlevel (usually about 400 psig) by a Grove back-pressure regulator placedon the product gas exit line from the reaction chamber. Exit gases fromthe reaction chamber were continuously withdrawn at a constant rateduring the reaction period and subjected to analysis by gaschromatography and mass spectrometry.

Various catalytically active metals and metal salt-based carriers weretested at a variety of different catalyst concentrations in continuousflow test runs which ranged from 3 to 24 in duration. The results ofthese tests including further descriptions of the catalyst systems andthe relative conversions to methane (usually ignoring carbon dioxide inthe exit gas, which introduces a slight error of about 10% in the 600°Cexperiments due to contribution from the CO-shift reaction) are given inTable I below.

                                      Table I                                     __________________________________________________________________________       Metal, Metal Oxide                                                            and/or Metal Carbide                                                                         Molten Salt  Catalyst                                       Run                                                                              Catalysts of Elements                                                                        Carrier      Precursor                                      __________________________________________________________________________    1   None (carborundum                                                                           None          --                                                   chips)                                                                 2   Manganese     ZnI.sub.2    MnI.sub.2                                      3   Iron          ZnBr.sub.2   Fe(CO).sub.5                                   4   Iron          ZnBr.sub.2   Fe(CO).sub.5                                   5   Iron          ZnBr.sub.2   Ferrocene                                      6   Iron          ZnBr.sub.2   Ferrocene                                      7   Iron          ZnBr.sub.2   Ferrocene                                      8   Iron          ZnCl.sub.2.sup.a)                                                                          FeCO.sub.5 +Ferrocene                          9   Molybdenum  5/1 CuBr/KBr   Mo(CO).sub.6                                   10  Iron        2/1 FeCl.sub.2 /KCl                                                                          Fe(CO).sub.5                                   11  Iron          SnCl.sub.2   Fe(CO).sub.5                                   12  Molybdenum  Li.sub.2 CO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3.sup                    .b)            Mo(CO).sub.6                                   13  Cobalt      Li.sub.2 CO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3.sup                    .b)            CoCO.sub.3                                     14  Iron        Li.sub.2 CO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3 /                      CaCO.sub.3.sup.c)                                                                            Ferrocene                                      15  Zinc          ZnBr.sub.2   Zinc Powder                                    16  Manganese   LiCO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3.sup.b)                                       MnCO.sub.3                                     17  Nickel        ZnI.sub.2    Nickel Powder                                  18  Nickel         ZnBr.sub.2  Nickel Powder                                  __________________________________________________________________________     .sup.a) ca. 4% by weight ammonia added to molten salt catalyst system.        .sup.b) eutectic mixture of alkali metal carbonates, i.e., LiCO.sub.3         /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3 at a weight ratio of 87/75/90.           .sup.c) eutectic mixture of alkali metal carbonates plus ca. 10% by weigh     CaCO.sub.3.  -                                                           

       Catalyst                                                                      concentration                                                                             Total                                                             %w elemental                                                                              weight                                                            metal based on                                                                            catalyst        % conversion                                      weight of catalyst                                                                        plus carrier                                                                          Reaction                                                                              of carbon monoxide                             Run                                                                              plus carrier                                                                              (grams) Temperature                                                                           to methane                                     __________________________________________________________________________    1    None       None   600°C                                                                             5                                           2    7          300    600°C                                                                            39                                           3    3          300    500°C                                                                            25                                           4    3          300    600°C                                                                            45                                           5    2          300    500°C                                                                            24                                           6    2          300    550°C                                                                            33                                           7    2          300    600°C                                                                            41                                           8    3          270    500°C                                                                            26                                           9    2          370    500°C                                                                            19                                           10   2          290    500°C                                                                            28                                           11   2          330    500°C                                                                            24                                           12   6          300    600°C                                                                            30                                           13   1          250    600°C                                                                            50                                           14   2          300    600°C                                                                            36                                           15   2          300    600°C                                                                            44                                           16   5          260    600°C                                                                            38                                           17   5          300    600°C                                                                            20                                           18   4          300    600°C                                                                            12                                           __________________________________________________________________________

EXAMPLE II

A series of batch or static methanation reactions were carried out withvarious molten salt-based catalyst systems according to the invention.These experiments were carried out utilizing a reaction system andprocedure which was similar to that described in Example I in allregards except the manner of reactant introduction and the analysisperformed on the reaction products. In these batch experiments thereactant charge (2:1 mole ratio of H₂ :CO) was charged into the reactoruntil a given pressure, i.e., 400 psig, was obtained and the system wassealed off. The rate of the methanation reaction in this sealed systemwas followed by observing the rate of pressure drop in the reactor whichresults from the conversion of four moles of starting material to twomoles of products. This reaction analysis method gives a good relativemeasure of the catalyst activity for methanation because the CO-shiftreaction which also occurs to a certain extent gives no change inpressure since the moles of product equal the moles of reactants. Theresults of this series of batch or static methanation reactions isrecorded in Table II below wherein relative ratings are given for theactivities -- e.g., high, medium or low -- of the catalysts evaluated.As a rule of thumb, a "high" activity rating was given to those catalystsystems which gave more than 50 percent conversion of CO to CH₄ in lessthan 10 minutes whereas the lowest rating, i.e., "very low" for activitymeans less than a 10 percent conversion over a reaction time of 1 houror more.

                                      Table II                                    __________________________________________________________________________       Metal, Metal Oxide                                                            and/or Metal Carbide                                                                         Molten Salt  Catalyst                                       Run                                                                              Catalysts of Elements                                                                        Carrier      Precursor                                      __________________________________________________________________________    19  Iron          ZnBr.sub.2   Fe.sub.2 (CO).sub.9                            20  Iron        LiCO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3.sup.a)                                       Fe.sub.2 (CO).sub.9                            21  Iron        FeCl.sub.2 /Nacl                                                                             Fe(CO).sub.5                                   22  Nickel        ZnBr.sub.2   Ni(C.sub.5 H.sub.5).sub.2                      23  Zinc          ZnBr.sub.2   ZnBr.sub.2 /Na.sub.2 CO.sub.3                  24  Iron          ZnBr.sub.2   Iron Powder                                    25  Manganese   MnCl.sub.2 /NaCl                                                                             MnCl.sub.2                                     26  Silver      LiCO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3.sup.a)                                       Ag.sub.2 O                                     27  Iron          LiCl/LiF     Fe(CO).sub.5                                   28  Titanium      ZnBr.sub.2   TiO.sub.2                                      29  Thorium       ZnBr.sub.2   ThO.sub.2                                      30  Thorium       FeCl.sub.2 /NaCl                                                                           ThO.sub.2                                      31  Iron          CaCl.sub.2 /LiCl                                                                           Iron Powder                                    32  Copper        FeCl.sub.2 /NaCl                                                                           Cu.sub.2 S                                     33  Iron          ZnBrhd 2     Fe(OH).sub.2                                   __________________________________________________________________________     .sup.a) eutectic mixtures of alkali metal carbonates, i.e., LiCO.sub.3        /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3 at a weight ratio of 87/75/90.      

     - Catalyst                                                                      concentration                                                                             Total                                                             %w elemental                                                                              weight                                                            metal based on                                                                            catalyst                                                          weight of catalyst                                                                        plus carrier                                                                          Reaction                                                                              Relative Activity                              Run                                                                              plus carrier                                                                              (grams) Temperature °C                                                                 for methanation                                __________________________________________________________________________    19   1.8        306     500,600                                                                              high                                           20   1.1        258     500,600                                                                              moderately high                                21   2.8        201      500   high                                           22   1.0        303      500   moderately high                                23   2.1        306      600   medium                                         24   1.8        306     500,600                                                                              high                                           25   26         190      600   low                                            26   7.9        272      600   low                                            27   1.8        306      600   high                                           28   1.6        305      600   moderately low                                 29   3.7        311      600   moderately low                                 30   5.6        205      600   moderately low                                 31   4.1        136      600   very low                                       32   1.6        202      600   low                                            33   0.9        303     500,600                                                                              high                                           __________________________________________________________________________

EXAMPLE III

A series of of absorption experiments performed to demonstrate theability of various molten salt carriers according to the invention toabsorb water at the high temperatures utilized in the methanationprocess according to the invention, i.e., 500°C and above and therebyshift the methanation reaction equilibrium towards methane formation. Inthese experiments the molten salts and water (as steam) were held in asmall 310 stainless steel autoclave equipped with an electric heater,thermowell and pressure gauge. Sealing was achieved through use of acopper gasket which was compressed upon heating and upon cooling as thevarious parts of the vessel expand and contract. Connecting lines to thepressure gauge were heated to prevent condensation anywhere in thesystem. After adding the salt and water to the autoclave, pressure wasrecorded as the apparatus was heated with occasional shaking to themaximum temperature desired, near 600°C. The apparatus was then allowedto cool and heated up again. This procedure helped to assure attainmentof equilibrium and allowed corrections to be made for gas producedthrough hyrolysis of salts or corrosion of the vessel. The extent ofwater absorption by the molten salt at equilibrium was obtained bycomparing the vapor pressure of the system with that which would bepredicted (for the quantity of water added) from Raoult's law (vaporpressure over ideal solution). The results are given in Table III.

                  Table III                                                       ______________________________________                                                      Total Water                                                                             % water absorbed by Salt                                            In System Phase at temperature (°C)                      Molten Salt   %m        425    475  525  575                                  ______________________________________                                        ZnBr.sub.2    10        92     90   87   83                                   ZnBr.sub.2    20        86     86   82   81                                   ZnBr.sub.2    30        87     85   81   77                                   LiCO.sub.3 /Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3                                              4        --     --   37   --                                   (Ternary eutectic)                                                            ______________________________________                                    

What is claimed is:
 1. A molten metal salt-based catalyst system which is active in promoting methanation at reaction temperatures above 500°C comprising a molten metal salt carrier selected from the class consisting of the halides and carbonates of alkali metals and alkaline earth metals and the halides of zinc, copper, manganese, cadmium, tin and iron, and mixtures thereof, melting below 1000°C; said molten salt having dispersed therein one or more catalytically active forms of iron selected from the class consisting of finely divided elemental iron, iron oxides, iron carbides or mixtures thereof.
 2. The catalyst system of claim 1, wherein the molten metal salt carrier melts below about 700°C.
 3. The catalyst system of claim 2, wherein the molten metal salt carrier melts between about 100° and about 600°C.
 4. The catalyst system of claim 1, wherein the average particle size of the dispersed catalytically active iron metal is no larger than about 0.5mm.
 5. The catalyst system of claim 4, wherein the concentration of catalytically active iron metal dispersed in the molten metal salt carrier ranges from about 0.1 to about 20% by weight of the total catalyst composition.
 6. The catalyst system of claim 5, wherein the molten metal salt carrier is selected from the class consisting of mixtures of alkali metal halides, mixtures of alkali metal carbonates and the halides of zinc, tin and iron, said iron halide having a minor amount of alkali metal halide added thereto as a melting point depressant.
 7. The catalyst system of claim 6, wherein the molten metal salt carrier is selected from the class consisting of zinc chloride, zinc bromide and zinc iodide.
 8. A molten metal salt-based catalyst system which is active in promoting methanation at reaction temperatures above 500°C comprising a molten iron halide carrier, said iron halide carrier having a minor amount of an alkali metal halide selected from the class consisting of sodium chloride and potassium chloride added thereto as a melting point depressant whereby the melting point of the resulting carrier is below 1000°C; said resulting iron halide carrier having dispersed therein one or more catalytically active metals selected from the class consisting of iron, molybdenum, manganese, nickel, cobalt, zinc, titanium, silver, copper and thorium in the form of finely divided elemental metals, metal oxides, metal carbides, or mixtures thereof.
 9. The catalyst system of claim 8, wherein the average particle size diameter of the dispersed catalytically active metal is no larger than about 0.5mm.
 10. The catalyst system of claim 9, wherein the concentration of catalytically active metal dispersed in the molten iron halide carrier ranges from about 0.1 to about 20% by weight of the total catalyst composition.
 11. The catalyst system of claim 10, wherein the catalytically active metal is selected from the class consisting of iron, zinc, manganese and molybdenum.
 12. The catalyst system of claim 11, wherein the catalytically active metal is zinc.
 13. The catalyst system of claim 11, wherein the catalytically active metal is iron.
 14. A molten metal salt-based catalyst system which is active in promoting methanation at reaction temperatures above 500°C comprising a molten metal salt carrier melting below 1000°C, said molten metal salt carrier being the terneary eutectic mixture of lithium, sodium and potassium carbonate; said molten salt having dispersed therein one or more catalytically active metals selected from the class consisting of iron, molybdenum, manganese, nickel, cobalt, zinc, titanium, silver, copper and thorium in the form of finely divided elemental metals, metal oxides, metal carbides, or mixtures thereof.
 15. The catalyst system of claim 14, wherein the average particle size diameter of the dispersed catalytically active metal is no larger than about 0.5mm.
 16. The catalyst system of claim 15, wherein the concentration of catalytically active metal dispersed in the molten metal salt carrier ranges from about 0.1 to about 20% by weight of the total catalyst composition.
 17. The catalyst system of claim 16, wherein the catalytically active metal is selected from the class consisting of iron, zinc, manganese and molybdenum.
 18. The catalyst system of claim 17, wherein the catalytically active metal is zinc.
 19. The catalyst system of claim 17 wherein the catalytically active metal is iron. 